April 12, 2019 — Replacing fish food with plants may not be as planet-friendly as it seems, according to a new study on the ecological impact of feeding soy and other land-grown crops to farmed seafood. These plant-based feeds are an alternative to, well, other sea creatures, which is what many species like shrimp and salmon eat in the wild. Published in the science journal Sustainability, the new study—which involved which involved an international and multidisciplinary team of experts—quantifies the effects that plant-based feeds have on land, water, and fertilizer use. The numbers that emerge challenge the prevailing notion that simply swapping fish-based fish food for plant-based fare can minimize the environmental footprint of aquaculture.

The interlocking limitations of fish-based feed, also known as fishmeal, have long confounded the aquaculture industry. For one, its ingredients—small, wild fish lower in the food chain, known as forage fish—are a finite resource. And as the global appetite for seafood continues to rise, so does the pressure to catch more. That means that fishmeal is becoming more costly and harder to source. As a result, producers have been trying to reduce their reliance in recent years.

“When shrimp farming became very popular 30 to 35 years ago, there was kind of a preference to grow as many shrimp as you can, as fast as you can, and get them out on the market,” says Cheryl Shew, a representative of specialty feed manufacturer Ziegler Bros. Inc. But like much of the industry, Ziegler began to experiment with soy as a partial substitute fishmeal in its feed products beginning in the early aughts, Shew tells me.

So why not just completely replace fishmeal with plants? It’s not as easy as it sounds—or as environmentally-sound, as it turns out.